1. Field of the Invention
The present invention relates to a DC voltage regulator and in particular to an error amplifier in the DC voltage regulator. This error amplifier advantageously includes a single compensation solution for both the voltage control loop and the current limit control loop.
2. Related Art
Typical DC voltage regulators employ a voltage control loop to stabilize output voltage over a variety of conditions. FIG. 1A illustrates a conventional DC voltage regulator (hereinafter regulator) 100. In regulator 100, an error amplifier 102 provides its output to an output driver 103. Output driver 103 typically comprises hundreds or even thousands of transistors. Voltage VOUT in turn drives a load 110, which has both a resistance component 105 and a capacitive component 106. In a typical embodiment, load 110 is located external to the integrated circuit implementing regulator 100.
In regulator 100, the voltage control loop includes resistors 108 and 109 connected in series between the node providing VOUT and a voltage source VSS. A node 111, which is located between resistors 108 and 109, provides a feedback voltage to the negative input terminal of error amplifier 102. A reference voltage Vref, which is typically generated by a bandgap circuit 101, is provided to the positive input terminal of error amplifier 102. In this configuration, error amplifier 102 can measure (i.e. sample) the output voltage VOUT using the resistive divider comprising resistors 108 and 109, thereby facilitating equalization of the input voltages to error amplifier 102. This voltage control loop is provided for normal operating conditions in regulator 100.
A standard regulator can also include another control loop for over-current conditions, which could be destructive to the regulator as well as the integrated circuit on which it is formed. For example, FIG. 1B illustrates an exemplary current limit circuit 104 that can be added to regulator 100 to form a modified regulator 100′. Current limit circuit 104 can include two PNP transistors 121 and 122 connected in series between a resistor 120 and an amplifier 123. Both PNP transistors 121 and 122 receive the output of power device 103 on their bases. Resistor 120 is connected between the emitter of PNP transistor 121 and a source voltage VCC. Amplifier 123 receives a first input from source voltage VCC, a second input from the emitter of PNP transistor 121, and a third input from the collector of PNP transistor 122.
Amplifier 123 provides its output via a feedback loop to the input of power device 103. A node between the collector of PNP transistor 121 and the emitter of PNP transistor 122 provides the output voltage VOUT when PNP transistor 121 is conducting. In this configuration, compensation circuit 104 can sense a current ISENSE at the output of power device 103. If a current limit condition exists, then amplifier 123 supplies a current that is proportional to the excessive current (i.e. the amount of current exceeding a nominal maximum current). This current should correspondingly limit the drive to power device 103, thereby preventing localized overheating on the integrated circuit.
Note that when the output voltage VOUT gets too high relative to an input voltage source VIN (e.g. VIN−VOUT<150 mV), PNP transistor 121 begins to saturate, thereby turning on PNP transistor 122. When PNP transistor 121 begins to saturate, excess carriers are injected into the substrate. These excess carriers in the substrate can undesirably cause significant instability in many components of regulator 100.
In one embodiment, current limit circuit 104 can detect and respond to both conditions. Specifically, in either case (or even potentially when both conditions exist), amplifier 123 can use its inputs to generate a current commensurate with the degree to which a current limit condition exists and/or when PNP transistor 121 begins to saturate. Note that amplifier 123 has two inputs: one voltage sensitive differential input for ISENSE (therefore, amplifier 123 can function as a gm amplifier) and one current sensitive single-ended input for PNP transistor 122 (therefore, amplifier 123 can also function as a current amplifier).
Unfortunately, either or both of the voltage control loop and the current limit control loop in regulator 100′ can be a source of regulator destabilization. Specifically, each control loop should provide a negative feedback in regulator 100′. Unfortunately, components in these control loops could undesirably cause a positive feedback, thereby destabilizing regulator 100′ by causing an oscillation in output voltage VOUT. This oscillation could prevent efficient voltage correction, elimination of a current limit condition, or could even cause significant damage to other integrated circuits driven by regulator 100′.
Therefore, a need arises for a system and method of providing stabilized control loops in a regulator.